The technical field to which this invention pertains is the creation of multilayered inorganic films which can react thermally to create structures for patterning layers applicable both to lithographic processes, such as those used in integrated circuit fabrication, and the making of images in thin films, such as is required in creating optical masks.
Integrated circuit and semiconductor devices are built using microfabrication lithographic techniques to pattern many layers of conductors, insulators or semiconductors. In lithography a masking layer, called a resist, is deposited on the device substrate and exposed by optically projecting an image onto its surface. For optically sensitive resists (photoresists) a chemical reaction changes the resist. Typically after development the areas exposed to the light will be removed, while those not exposed will remain creating a raised pattern of resist on the surface (a reverse or negative resist process is also possible). This raised pattern protects parts of layers below so that when exposed to an etching environment (for example acids, etching gases or plasmas or ion beams) the areas under the remaining resist are protected from etching, while those without resist are preferentially removed. This resist layer is then striped or removed leaving a transferred raised pattern from the mask in the layer on the substrate. The patterned layer may be used directly as defined or in turn may be used to pattern a layer below it on the substrate, either for additional etching processes, or other operations (for example creating doped patterns with impurities, or growing patterned oxide layers). Repeated processes of layer deposition and resist lithographic patterning are used to create everything from simple single layer structures to complex integrated circuits.
Indeed the very photomasks used in the optical lithographic process are created by directly writing with a focused laser or electron beam spot a pattern into a resist on an optically transparent substrate, usually coated with a thin absorbing layer. That resist pattern then defines the etching of the lower layer, patterning the absorbing and non-absorbing areas on the transparent substrate creating the mask used in other lithographic processes.
Current lithographic processes typically use organic based photoresists which are applied as liquids to a substrate or wafer which is then spun at high speeds so that interaction of rotational, gravitational forces, surface tension and viscosity creates a controlled thickness of resist. The film is then baked to remove solvents before the photolithographic exposure. The photoresist is then developed using a wet chemical processes that dissolves the unwanted resist away. After the lithographic etching processes the photoresist is stripped (often in an oxygen plasma etcher or with liquid strippers). However it is very hard to remove all the organics added by the resist, and there is always the danger of other outside contaminants. Hence very aggressive chemical cleans, such as the industrial standard RCA clean, must be used to remove these organics. This combination of steps: cleaning, deposition or growth of a layer, photolithographic definition, etching and resist stripping is repeated for each layer and pattern to make the final circuit of even the simplest device. These cleaning processes are very time, energy and material consuming. Resist contamination left behind is a common source of defect creation in integrated circuit processes.
In contrast to the wet organic photoresists and their related cleanups most other processes in modern microfabrication are dry, often vacuum based procedures. Many types of deposition (plasma sputtering, Chemical Vapour Deposition (CVD), ion implant) and etching processes (Plasma, Reactive Ion Etching (RIE)) use low-pressure techniques that introduce much fewer contaninants into the processes.
In addition to contamination problems organic resists are very wavelength sensitive. Current optical exposure systems use Ultra Violet (UV) Excimer lasers operating at 248 nm wavelength as the light source, producing short (5-20 nsec.) pulses of high power for exposure to create the small structures needed. Resists for the current 248 nm wavelengths will not work for the future generations of exposure systems which currently plan to use 193 nm, 150 nm or even shorter wavelengths to make structures smaller than 0.1 microns. Furthermore, at those shorter wavelengths and high power pulses many organic resist materials are damaged (photoablated) because the energy of the UV light tears apart the molecules of the organics. This photoablation can cause problems with materials deposited on the exposed optics.
These problems have suggested that a switch to inorganic based dry resist processes would provide significant advantages. Firstly, a dry resist process would permit devices to be fabricated mostly in a vacuum based environment, allowing transfer from a dry based deposition (for example sputter deposition) to the dry inorganic resist coating, to the exposure, etching (say plasma etching) to the resist stripping processes. This would keep devices much cleaner, offering less source of contamination, and hence potentially reducing the rate of defects. Secondly the removal of the organics from the resists may significantly reduce the number of cleans needed in process steps with savings in time, materials and energy. Thirdly many organic resists are thermally activated, that is the optical exposure creates a local temperature rise, which in turn creates the inorganic reaction for the development. Thermal resists, especially those using metal-based inorganics, can be less wavelength sensitive and operate at very short wavelengths. Fourthly metal-based inorganics can avoid the photoablation effect down to very short wavelengths. Fifthly thermally reacted inorganics can show different optical characteristics after exposure than before. Thus the exposed areas can be identified before the development processes. This allows errors in exposure to be corrected.
Finally Gelbart and Karasyuk have shown that with thermal resists and a special multiple exposure modification existing optical exposure systems could substantially decrease the minimum size structures they can build. Current exposure systems are diffraction limited by their optics and need to use shorter wavelengths (193 nm and 150 nm) to pattern structures below 0.1 to 0.07 microns. However Gelbart and Karasyuk show that by use of a multiple exposure modification to existing (248 nm) systems resolution below 0.1 microns may be possible and below 0.1 micron with shorter wavelengths. This multiple exposure system only works for resists that do not follow the law of reciprocity. The law of reciprocity says that total exposure is integrated over time, meaning that two exposures at half-threshold have the same outcome as one exposure at full threshold; in either case the resist will be fully developed. Thermal resists react when the resist is heated above a certain temperature and do not follow reciprocity. Thus, if a thermal resist is heated to just below the threshold, allowed to cool, and then heated a second time to the same point, it will remain unexposed. In a microfabrication exposure system, UV light arrives in pulses of a few tens of nanoseconds spaced hundreds of microseconds apart. This means that there is sufficient time for the material to cool between UV exposures. By comparison standard photoresists follow the law of reciprocity and a multiply exposure system produces the same result as a regular exposure.
While thermal inorganic resists offer these advantages previous art has shown these resists have, until now, had significant problems, especially with their sensitivity. Janus in U.S. Pat. No. 3,873,341 proposed an amorphous iron oxide based film as a thermal resist. When heated by the optical exposure system the amorphous iron oxide is crystallized if the local temperature exceeds 820xc2x0 C. The crystallized iron oxide areas are attacked by acid more rapidly than the amorphous, and hence can be selectively removed. However, this high threshold temperature would require unacceptable exposure light intensities in current optical exposure systems.
Bozler et al. in U.S. Pat. No. 4,619,894 offers another thermal inorganic resist consisting of an aluminum film deposited in a low-pressure oxygen atmosphere. This creates an aluminum oxide cermet. When exposed to a UV laser pulse the cermet is converted from a conductive phase to a highly resistive oxide phase. This resistive material is also etched at a much lower rate by a phosphoric acid etch than the cermet, thus creating the desired resist structure. While the optical exposure requires by the cermet resist is 1000 times less than the Janus resist, it is still requires temperatures in the 300xc2x0 C. range, and thus exposures 4-10 times greater than current resists, requiring 40 to 100 mJ per square cm of UV light in the 20 nsec laser pulse. Current resists require about 10 mJ per square cm per pulse of UV light for exposure.
Using lasers to alloy film layers and alter their reflectivity is well known in the creating of optical writeable disks for information storage. In particular Takeuchi et al. in U.S. Pat. No. 5,851,729 describes a system using a Bismuthxe2x80x94Tellurium Bixe2x80x94Te alloy layer that is sandwiched between two Antimonyxe2x80x94Selenium Sbxe2x80x94Se films. When hit with a laser the 3 layers alloy to create a Bixe2x80x94Texe2x80x94Sbxe2x80x94Se film with different reflectance than that of the unalloyed layer. Nakane in U.S. Pat. No. 4,587,533 teaches another optical write alloying system using a transparent layer with a lower melting temperature than the metal layer of Te, Bi, Sb or In. The laser light passes through the upper layer, and melts the lower which alloys with the transparent upper layer to significantly change the materials reflectivity. However while Takeuchi, Nakane and others teach the multilayer alloying concept they focus on creating alloys whose optical characteristics, especially their reflectivities, substantially change from the unexposed to alloyed state. The relative etching rate of the unexposed and alloyed areas is not considered or discussed as important in their choice of materials. For a workable thermal resist for microfabrication, the change in the optical characteristics is of much less importance than the etch ratio of the unexposed and alloyed films. Optical changes would be useful to identify the exposed area before development, but it is not necessary for a successfully functioning thermal resist. Optical changes may be useful in some applications like creating photomasks but should not be the most important issue for thermal inorganic resists. In addition the optical writeable disk technologies all require that the unexposed multilayers not be alloyed by the laser beams reading the disk information. Since these reading lasers are typically 1-2 mW of laser light focused to spots of less than one micron this requires a significant thermal threshold (typically more than 250-400xc2x0 C.) before the alloying can occur. However good thermal resists require the opposite condition: the lower the thermal reaction temperature the more sensitive to illumination is the resist making it more desirable. Indeed the exposure induced by the optical writable disk readout laser is typically in the 400,000 W/sq cm range making it unacceptable in current photolithographic exposure systems. Thus the alloys and processes best for optical disk writing are poor candidates for thermal inorganic resists just on thermal energy considerations.
The Janus and Bozler thermal inorganic resists are still too insensitive for most applications. In addition they are specific to particular materials and thus hard to modify to improve the process. Accordingly what is needed is a more general way of creating inorganic resists that offers a range of materials that can be explored for resists that have both good sensitivity (UV exposure requirements) and are compatible with current integrated circuit contamination requirements. In addition some optical characteristics of some inorganic resists in this class may make them useful for creating optical masks directly if their light absorption can be changed from a highly absorptive state to a less absorptive one. This innovation offers the potential for addressing all of these needs.
This invention discloses a general class of dry inorganic thermal resists based on a multilayer process. In its simplest form the thermally active layer consists of a lower thin film of one inorganic material, usually single element metals of binary metal alloys (in one example an Indium (In) film). This film is deposited via a dry process (for example sputter deposition or CVD deposition). Usually in the same deposition system a second layer of another inorganic film is deposited, again commonly a single element metals or binary metal alloys (in one example a Bismuth (Bi) film). In more complex forms there may be more than two layers. The materials that are potential candidates for the two or more layer films are those that show in their phase diagrams a low temperature alloy, a eutectic, whose melting point is below that of the two individual films. Best results occur for eutectic temperatures below 300xc2x0 C. and preferentially below 200xc2x0 C., with temperatures of 100xc2x0 C. or below being possible with some alloys. Some phase diagrams show more than one phase material ratio with temperatures below the individual layer melting points. Thus, when heated by optical exposures that create temperatures in the film above the eutectic point, the films begin to alloy at the interface between the layers. Since the alloy has a lower melting point than either individual film, this reaction will occur at a much lower temperature than the melting or vaporization temperatures of the individual films. None of the films tested show ablation of the materials after exposure at levels near the threshold. The ratio of the film thicknesses must be such that when fully combined the resulting alloy will near the desired composition. Since current laser optical UV microlithographic exposure systems involve very short pulses (about 20 nsec or less) the relatively low average energy of exposure actually involves very high instantaneous optical powers during a single pulse (10 mJ per square cm in 20 nsec. is 0.5 megawatts of power per square cm during the pulse). Such high powers drive the alloying process if the film is thin enough.
In addition to the low temperature eutectic point the films should show important thermal and optical characteristics. Both materials must have a sufficiently low thermal conductivity that the heated area does not create temperatures in adjacent areas to exceed the threshold temperature during and after the laser pulse (given tile pulse to pulse variation in exposures that occur in typical systems and thus the required exposures above the threshold levels to produce uniform results). The area over which the heat would spread will determine the resolution of the film, and for current applications should allow resolutions of less than 0.1 microns. In addition the lower the thermal conductivity the less energy needed to heat the local area above the thermal reaction point.
Optical absorption characteristics of the films also determine the combined layer of thickness and the order of the layers. The order of which material is on the top layer is often set to give a minimum reflection and maximum absorption with the film. For proper choice the material""s optical index of refraction and absorption index at the desired wavelengths should be calculated to obtain these. Total film thicknesses are best when the optical energy absorption rate allows energy deposition in both film layers. As many films are highly absorbing metals calculations must be done using optical multilayer thin film analysis including complex indexes, and Poynting Vector analysis, both including the effects of internal reflections. In general the actual energy deposited in the film per unit volume increases as thickness decreases, resulting in more sensitivity for total film thickness less than 70 nm. The films are in general only modestly wavelength sensitive. For some metal films the wavelength range of successful operation tested has ranged from the Near Infrared (860 nm) to UV (266 nm). There are however different exposure requirements at different wavelengths. While films less than 70 nm provide best sensitivity thicker films (up to at least 300 nm) show the same alloying effects, but required ore laser power.
While an alloying action is the simplest, some alloys may also combine with oxygen, nitrogen or hydrogen to form films with significantly different characteristics than both the thin layer materials or their oxides, nitrides or hydrides. This may occur either with gases in the atmosphere above the films during exposure, or from atoms trapped in the film during deposition, or even from layers that are oxides to begin with (usually the bottom layer).
The post-alloying characteristics needed are dependent on the application. Most useful materials show a significant optical change between the alloyed and non-alloyed areas. Depending on the materials used alloyed films range from little optical change to significant increases/decrease in reflectivity or color to films. Optical transmissions may significantly decrease, and in some tested cases actually go from nearly completely absorbing in the unexposed case to almost completely transparent in the alloyed case (as in the case of one embodiment, BiIn, films showing such changes). Films that alloy and show a substantial decrease in absorption may be used to directly write optical images photomasks for some applications.
The development etching characteristics of the alloyed layer in some materials are very different from the unalloyed material. General results have shown the alloyed material is much more resistant to etching than the unexposed layers. Etching rate ratios of alloyed to unexposed will depend on the specific etchant and etching processes used (for example wet acids or dry plasma). With some alloys (as in BiIn) a simple wet etchant will remove the unalloyed layer, leaving the alloyed layer. This development etch generally results in a negative thermal resist with the alloyed areas left behind and the unexposed areas removed.
The thermal inorganic resist layers are very thin (typically 30-70 nm). Also they are not resistive to all etchants needed for all commonly used layers on the substrate. In most applications an inactive protection layer is deposited before the multilayer resist is deposited. After the patterning exposure and development etch of the multilayer inorganic resist it acts as a mask layer for the protection layer. Protection layers are chosen to have a significantly different etch characteristic to that of the alloyed resist. For example in the case of metal multilayers a thick (about 1 micron) layer of carbon would be preferred. An oxygen plasma development etch would in many cases remove the carbon without attacking the alloyed resist. This leaves a multilayer developed resist consisting of the alloyed resist and protection layer. This provides significantly better protection for etching many of the lower layers.
To work in any microfabrication process a resist must be easy to strip or remove, leaving the substrate clean of any remnant resist or contamination, but without damaging any of the layers below this is especially important for reworking a resist definition that has defects in it. A microfabrication standard cleaning processes etch (RCA 2 or HCI: H2O2: H2O in the ratio 2:3:14) has proven successful in stripping the thermal inorganic resist in one case (BiIn). Dry etches or ion milling can also be used as the layer is very thin. The protection layer can be removed with a dry etch (Oxygen plasma in the case of a carbon layer). This will leave a clean substrate ready for the next processing step. It is notable that neither the Janus nor the Bozler patents discuss the strip process for their resists. Removal of the patterned cermet resist in Bozler case without damage to some metal layers below would be difficult.
Since more than one material combination has successfully shown some or all of these characteristics this patent discloses a general class of inorganic thermal resists that have low temperatures of conversion, form alloys over a wide wavelength range, do not show ablation of materials after exposure, show significant different optical parameters from the unexposed areas after the exposure allowing the patterning to be determined before development, more than one material is transparent after exposure making them candidates for optical masks, and some show a significant etching difference between the alloyed and unexposed materials allowing a development etch to pattern the layer, and yet have a simple resist strip.